Enhanced quality factor resonator

ABSTRACT

A high quality factor resonator (15) including a substrate (110) and a resonator layer (150). The resonator layer includes a first electrode (157). The resonator layer (150) is disposed on a surface of the substrate (110). The first electrode (157) is disposed on a distal surface of the resonator layer (150). A cavity (120) is disposed between the substrate (110) and the resonator layer (150) and a second electrode (159) is disposed on the substrate (110) and on a surface of the cavity (120) remote from the resonator layer (150) and is separated from the resonator layer (150) by a thin gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent applicationshaving Ser. Nos. 08/496,837 and 08/496,835, filed on an even dateherewith and which are assigned to the same assignee as the presentapplication.

FIELD OF THE INVENTION

This invention relates in general to the field of frequency selectioncomponents, in particular to bulk acoustic wave frequency selectioncomponents and more particularly to an improved quality factor acousticwave frequency selection component.

BACKGROUND OF THE INVENTION

Frequency selective components are important for many electronicproducts requiring stable frequency signals or ability to discriminatebetween signals based on frequency diversity. These functions aredifficult to reliably and repeatably realize in monolithic form togetherwith other microelectronic components such as transistors, diodes andthe like.

One approach to realizing frequency selective functions employs a massallowed to vibrate in one or more dimensions (e.g., a pendulum). Such amass is conveniently realized as a thin membrane supported at criticalpoints, for example, peripherally or alternatively along one edge orend, forming a thin resonator structure. Such structures provide clearlydefined mechanical resonances having significant utility, for example,as filters and as frequency stabilizing feedback elements in oscillatorcircuits. These structures have the advantages of being extremelycompact and of providing narrow bandwidth (i.e., high quality factor)frequency selection components that are light weight and which do notrequire adjustment over the life of the component.

Thin film resonators incorporate a thin film, free-standing membrane.Typically, this is effected by forming a sacrificial layer followed bydeposition of the membrane. The sacrificial layer is then selectivelyremoved, leaving a self-supporting layer.

An alternative approach involves forming a cantilevered beamcapacitively coupled to adjacent structures (e.g., a conductor placedbeneath the beam). The beam is free to vibrate and has one or moreresonance frequencies. Disadvantages of these approaches include need toform free-standing structures and also a tendency of the beam to "stick"to adjacent structures if or when the beam comes into contact therewith.

Problems encountered with such devices include reduced Q or qualityfactor due to at least two causes: (i) reduced quality factor ofmaterials employed for the piezoelectric element and (ii) reducedquality factor for the composite resonator owing to the contributions ofthe metallizations forming the electrodes. Additionally, higher couplingpiezoelectric materials (e.g., LiNbO₃, LiTaO₃, lithium tetraborate,ALPO₄, BiGeO₂₀, BiSiO₂₀ and the like) are preferred for someapplications but tend to be more difficult to realize in thin film form,especially as oriented films exhibiting significant piezoelectricity.

The Q of the material(s) employed in the resonator may precludeproviding the required bandwidth and insertion loss in the completedstructure. Generally, narrow bandwidths require high Q materials.Deposited thin-film layers of piezoelectric materials tend to havepoorer (i.e., lower) quality factors than the same materials prepared byother techniques (e.g., single-crystal materials) and this may limit theachievable bandwidth. Additionally, employing lossy materials forelectrodes (e.g., Au, Ag, Pb etc.) reduces the overall Q of theresonator structure while use of low acoustic loss materials (e.g., Aland alloys thereof) has less of an adverse impact on the Q of theresonator structure. Accordingly, the bandwidth requirements for someapplications may preclude use of some materials in the resonator and mayrequire the use of other materials or particular material preparationtechniques.

Many applications require robust, light-weight devices to be realized insmall form factor and to consume as little electrical power as possiblewhile operating over a broad range of temperatures. For example,satellite communications apparatus have stringent power requirements andalso must operate over a broad temperature range. This example alsoplaces a premium on size, weight and reliability.

What are needed are apparatus and methods for forming apparatus whereinthe apparatus provides a small, lightweight and robust resonator havingsolid mechanical support and including high quality factor,narrow-bandwidth frequency selection characteristics together with lowpower consumption requirements and low insertion loss.

SUMMARY OF THE INVENTION

A high quality factor resonator includes a substrate having a surfaceand a resonator layer including a first electrode. The resonator layerdesirably comprises a single-crystal material and is disposed on thesurface. The first electrode is disposed on a distal surface of theresonator layer. The resonator also includes a cavity disposed betweenthe substrate and the resonator layer and a second electrode disposed onthe substrate and on a surface of the cavity remote from the resonatorlayer, wherein the second electrode is separated from the resonatorlayer by a thin gap.

The resonator layer usefully comprises lithium niobate, lithiumtantalate or lithium tetraborate and preferably comprises single-crystallithium niobate, lithium tantalate or lithium tetraborate. In apreferred embodiment, the first electrode includes aluminum anddesirably is circular. The second electrode optionally comprises gold.

The substrate usefully includes a semiconductor substrate comprising anintegrated circuit. In one preferred embodiment, the resonator layer iswelded to the substrate.

A method for manufacturing a high quality factor resonator usefullyincludes steps of providing a substrate, providing a resonator layerhaving substantially parallel first and second surfaces and coupling theresonator layer to the substrate such that the resonator layer isdisposed atop the substrate and including a cavity disposed between thesecond surface of the resonator layer and the substrate.

The method desirably further includes a step of disposing a firstelectrode having a periphery on the first surface of the resonatorlayer, wherein the periphery is disposed over the cavity and within aperiphery of the cavity and the first surface is more remote from thesubstrate than the second surface.

The coupling step desirably but not essentially includes a step ofproviding a second electrode contained within the cavity. The secondelectrode is disposed parallel to the second surface and separatedtherefrom by a gap having a width in a range of from 0.05 micrometers toabout one micrometer.

The step of providing a substrate preferably includes a step ofproviding a semiconductor substrate including integrated circuitry. Thestep of providing a resonator layer preferably includes a step ofproviding a resonator layer comprising single-crystal material. The stepof providing a resonator layer usefully includes a step of providing aresonator layer comprising material chosen from the group consisting oflithium niobate, lithium tantalate and lithium tetraborate.

In a first preferred embodiment, the coupling step includes steps ofdisposing metal on a portion of the second surface of the resonatorlayer, disposing metal on a portion of the substrate, aligning the metalon the portion of the second surface with the metal on the portion ofthe substrate and welding the resonator layer to the substrate. Thewelding step desirably but not essentially includes a step of resistiveheating of the metal on the portion of the second surface and the metalon the portion of the substrate.

The method optionally includes a step of thinning the resonator layerfollowing the coupling step. The thinning step includes a process chosenfrom the group consisting of mechanical lapping, sputtering and ionmilling.

A high quality factor resonator comprising a substrate comprising asemiconductor material and including an integrated circuit and aresonator layer comprising a piezoelectric material and including afirst electrode. The resonator layer is disposed on a first surface ofthe substrate. The resonator also includes a cavity disposed between thesubstrate and the resonator layer and a second electrode disposed on thesubstrate and on a surface of the cavity remote from the resonatorlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified side view, in section, of a thin-filmacoustic resonator in accordance with the prior art;

FIG. 2 illustrates a simplified side view, in section, of a firstembodiment of a resonator in accordance with the present invention;

FIG. 3 illustrates a simplified side view, in section, of a secondembodiment of a resonator in accordance with the present invention; and

FIG. 4 illustrates a simplified plan view of a first embodiment of atemperature compensated resonator in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

Resonators in accordance with the present invention may be formed inseveral types of structures. These include a via hole structure and acavity structure. Both structures include placing a resonant structureon a surface of a substrate.

VIA HOLE STRUCTURE

FIG. 1 illustrates a simplified side view, in section, of acousticresonator 5 comprising substrate 11 and piezoelectric resonator layer 14having electrodes 17 and 19 and thickness 12 in accordance with theprior art. Thickness 12 is typically chosen to be one-half of anacoustic wavelength or an odd multiple thereof. A portion of resonatorlayer 14 disposed over via or cavity 13 and beneath electrode 17provides mechanical oscillations that determine the frequency responseof resonator 5.

Substrate 11 provides mechanical support for piezoelectric resonatorlayer 14 and electrodes 17 and 19 and for any ancillary components suchas transistors, diodes, capacitors, resistors and the like included aspart of a larger microelectronic device or integrated circuit (notshown). Substrate 11 conveniently comprises semiconductor material (s)or is compatible therewith (e.g., silicon on sapphire, cadmium sulphideon glass etc.). Semiconductor materials presently of particular interestinclude elemental semiconductors such as diamond, silicon, germanium andsilicon carbide, III-V materials such as gallium arsenide, aluminumnitride, indium phosphide and the like, II-VI materials such as cadmiumsulphide, zinc oxide and so forth and alloys such as, by way of exampleand not intended to be limiting, Si_(x) Ge_(1-x), Al_(x) Ga_(1-x) As andIn_(x) Al_(1-x) P. Cubic semiconductors (e.g., Si, Ge, GaAs etc.) areoften prepared as wafers having a [100 ] surface, for example, whichsurface is polished and otherwise prepared for use as semiconductordevice substrates. Other useful orientations include [110 ] and [111]faces.

An acoustic resonator such as 5 of FIG. 1 including substrate material11 having layers 14, 17, 19 disposed thereon is etched from the backside to provide an opening 13 extending up to the bottom of the membranecomprising layers 14, 17, 19. This can be accomplished by use ofetchants having etch rates sensitive to doping of semiconductormaterials 11 coupled with use of a surface layer of material havingdifferent doping than the bulk of the material, for example. Otheroptions include employing a surface layer or layers (e.g., layers 14,17, 19) of different composition and/or crystallographic form ororientation to provide a layer following etching or other treatment toselectively remove some of material 11 immediately therebelow.

Problems encountered with this structure in some applications include orstem from (i) limitations on the availability of high Q materials inthin film form, (ii) limitations on the Q of the resonator structure dueto the metallization (i.e., layers 17, 19) and (iii) limitations on thecoupling coefficient of the piezoelectric materials that this structurerequires.

CAVITY STRUCTURE

FIG. 2 illustrates a simplified side view, in section, of a firstembodiment of high quality factor resonator structure 15 whereinresonator layer 150 is supported above cavity 120 in the surface ofsubstrate 110 by thin layer of material 160. Cavity 120 also includeselectrode 159 below, but not in physical contact with, resonator layer150.

FIG. 3 illustrates a simplified side view, in section, of a secondembodiment of high quality factor resonator structure 15 whereinresonator layer 150 is supported above a cavity 120 disposed above thesurface of substrate 110 by a thin layer of material 160. In this case,cavity 120 is formed exclusively by layer 160.

In the structures depicted in FIGS. 2, 3, the gap separating electrode159 from resonator layer 150 is desirably small, on the order ofone-tenth or less of the thickness of resonator layer 150, but may belarger in some applications. Resonator layer 150 usefully has athickness in a range of from less than a micrometer up to about amillimeter, desirably in a range of from less than a micrometer up toabout ten micrometers and preferably in a range of from about one-halfof a micrometer to about five micrometers. Thicknesses as discussedherein are measured along an axis as indicated by arrow 152. Arrow 152also denotes the thickness of the combination of top electrode 157 andresonator layer 150. Substrate 110 is usefully a semiconductor substrateand desirably includes integrated circuitry.

Two elements of high quality factor resonator structure 15 increase thequality factor above that obtainable by prior art techniques: (i)utilizing single-crystal material for resonator layer 150 and (ii)separating resonator layer 150 from bottom electrode 159 by the smallgap therebetween. In the embodiment illustrated in FIG. 3, thisthickness is equal to the thickness of thin layer of material 160. Thesingle-crystal material employed for resonator layer 150 desirably butnot essentially comprises a dielectric material having a large relativepermittivity (ε_(r)) and also desirably comprises a piezoelectricmaterial (e.g., LiNbO₃, LiTaO₃, lithium tetraborate and the like). Thesecond element avoids viscous losses encountered in thin metal films atthe frequencies of interest when the metal films are insonified by theresonator structure 15. Because bottom electrode 159 is not in physicalcontact with resonator layer 150, bottom electrode 159 may comprisematerials (e.g., gold, silver etc.) that ordinarily would not besuitable for use as an electrode for exciting mechanical oscillationand/or resonance in resonator layer 150. Top electrode 157 is disposedon a distal surface of resonator layer 150, i.e., remote from substrate110, and desirably comprises a thin (e.g., in the range of from 0.05micrometers to several micrometers) layer of aluminum to reduce acousticlosses and mass loading effects. Typically, thinner films are employedfor top electrode 157 when thinner resonator layers 150 are employed inresonator 15.

A variety of techniques applicable to acoustic resonator structures 5,15 of FIGS. 1-4 are described in U.S. Pat. No. 4,556,812, G. R. Kline etal., "Acoustic Resonator With Al Electrodes On An AlN Layer And Using aGaAs Substrate" (Dec. 3, 1985); U.S. Pat. No. 3,313,959, J. G. Dill,"Thin-Film Resonance Device" (Apr. 11, 1967); U.S. Pat. No. 4,456,850,T. Inoue et al., "Piezoelectric Composite Thin Film Resonator" (Jun. 26,1984); U.S. Pat. No. 4,502,932, G. R. Kline et al., "Acoustic ResonatorAnd Method Of Making Same" (Mar. 5, 1985); U.S. Pat. No. 4,460,756, J.S. Wang et al., "Method Of Making A Piezoelectric Shear Wave Resonator"(Feb. 3, 1987); U.S. Pat. No. 4,642,508, H. Suzuki et al.,"Piezoelectric Resonating Device" (Feb. 10, 1987); U.S. Pat. No.4,719,383, J. S. Wang et al., "Piezoelectric Shear Wave Resonator AndMethod Of Making Same" (Jan. 12, 1988); U.S. Pat. No. 5,011,568, S. D.Brayman et al., "Use Of Sol-Gel Derived Tantalum Oxide As A ProtectiveCoating For Etching Silicon" (Apr. 30, 1991); U.S. Pat. No. 5,075,641,R. J. Weber et al., "High Frequency Oscillator Comprising Thin FilmResonator And Active Device" (Dec. 24, 1991); U.S. Pat. No. 5,162,691,E. A. Mariani et al., "Cantilevered Air-Gap Type Thin Film PiezoelectricResonator" (Nov. 10, 1992); and U.S. Pat. No. 5,373,268, L. N. Dworskyet al., "Thin Film Resonator Having Stacked Acoustic ReflectingImpedance Matching Layers And Method" (Dec. 13, 1994), which patents arehereby incorporated herein by reference.

HEATING ELEMENT

FIG. 4 illustrates a simplified plan view of temperature compensatedresonator 15 in accordance with the present invention. Temperaturecompensated resonator 15 includes electrode 157 disposed on a portion ofresonator layer 150 having a periphery that is within the boundaries ofvia 120, shown in dotted outline in FIG. 4. Electrode 159 is illustratedas covering the entire bottom of resonator layer 150 but need onlyextend to cover that portion of resonator layer 150 intended tocontribute to mechanical oscillation of temperature compensated acousticresonator 15. In the example illustrated in FIG. 4, electrode 159 isalso employed to mechanically affix resonator layer 150 to substrate 110through a process, for example, of resistive heating (§ IV, infra) orthermosonic bonding (§ V, infra).

FIG. 4 also illustrates transmission line 210 having electricalconnection to electrode 157 and extending from electrode 157 to externalelectrical circuitry (not illustrated). Transmission line 210 isconstructed to have a particular characteristic impedance by varying thewidth of transmission line 210 and taking into account the thickness andthe relative dielectric constant of resonator layer 150 and relatedstructures in accordance with principles well known in theelectromagnetic arts.

FIG. 4 further illustrates conductor 215 comprising a resistive loophaving interconnections 216, 216'. Conductor 215 is heated by current(e.g., DC) introduced via interconnections 216, 216' to maintainresonator layer 150 at a desired predetermined temperature. Generally,this is done in order to reduce or avoid thermally-induced drift of thefrequency response of resonator 15. Interconnections 216, 216' andtransmission line 210 are routed to provide appropriate electricalconnections to each, for example, by including an airbridge or othertype of crossover or by making conductor 215 in the shape of a reentrantloop (not illustrated in FIG. 4).

It will be appreciated that the structures of conductor 215 illustratedin FIG. 4 may be realized by disposing the material comprising conductor215 in other geometric shapes, e.g., in a substantially hexagonal oroctagonal shape, or as a circle or the like. Similarly, the shapes ofelectrodes 159, 157 need not comprise squares as illustrated but mayalso be other polygonal or curvilinear shapes. Moreover, the shapeadopted for electrode(s) 157 and/or 159 need not be the same as that ofconductor 215, e.g., a square shape may be adopted for electrode 157while a circular shape may be adopted for conductor 215. Conductor 215is desirably formed of a material such as TiW, NiCr, Cr etc. having athickness and linewidth to provide a desired resistance in accordancewith a typical prime power requirement of 100 milliwatts in order tomaintain a constant temperature over an ambient temperature range of100° C. In the preferred embodiment, electrodes 159, 157 and conductor215 desirably form circular shapes because these shapes reduceundesirable temperature gradients within the composite resonatorstructure.

RESISTANCE WELDING ATTACHMENT

Attachment of resonator layer 150, especially resonator layers 150comprising single-crystal materials such as LiNbO₃, LiTaO₃ and the like,may be effectuated via resistance welding, as described in "Fabricationof Wideband Bragg Cells Using Thermocompression Bonding and Ion BeamMilling", J. Rosenbaum et al., IEEE Trans. Son. Ultrason., Vol. SU-32,No. 1, January 1985, which article is hereby incorporated herein byreference. This article describes attachment and subsequent thinning ofsingle-crystal, high coupling coefficient materials. The two surfaces tobe bonded are metallized by vacuum evaporation to include 0.1 to 0.5micrometers of metal, preferably silver or gold. A pressure of 2000 to7000 lbf/in² (140 to 500 kg/cm²) is applied, and a resistance checkprovides positive identification of samples lacking intimate contactbetween the two surfaces to be joined. A single current pulse (e.g.,having an amplitude of tens of amperes and a duration in a range ofseveral tens of milliseconds) is applied to heat bonding film 160 to200° to 400° C. The specific current and duration of the pulse arechosen in accordance with the thermal properties of the materials to bejoined and thermal design principles.

Following attachment, the single-crystal piezoelectric material (e.g.,resonator layer 150) is mechanically lapped to a thickness of about 25micrometers and is subsequently ion milled to a thickness in a range ofone to three micrometers. Greater thicknesses may be employed for lowerfrequency devices.

ULTRASONIC WELDING ATTACHMENT

Attachment of single-crystal materials by ultrasonic welding isdescribed in "Performance of Single-Crystal LiNbO₃ Transducers OperatingAbove 1 GHz", by N. Uchida et al., IEEE Trans. Son. Ultrason., Vol.SU-20, No. 3, July 1973 or "Ultrasonically Welded PiezoelectricTransducers", by J. D. Larson III et al., IEEE Trans. Son. Ultrason.Vol. SU-18, No. 3, July 1971, which articles are incorporated herein byreference. These articles describe mounting of single-crystal materialshaving thicknesses in a range of more than one millimeter to about onehundred micrometers (with subsequent mechanical polishing to reduce thethickness to about six to ten micrometers) using pressures in a range offrom 2000 to 4000 lb/in² (140 to 280 kg/cm²), acoustic powers of lessthan a watt (18 kHz), temperatures in a range of from 250° to 400° C.and times of from 15 to 120 minutes (Larson et al.) or pressures in arange of 120 kg/cm², a few watts of ultrasonic energy (15 kHz),temperatures of about 300° C. and a time of about 90 minutes (Uchida etal.).

Subsequent thinning of the mounted resonator may be effected bysputtering as described by Uchida et al. or Larson et al.

EXAMPLE I

An example of a resonator design is summarized below and compared to aconventional design in Table I. The new design has about one-half themotional capacitance but about three-fourths the static capacitance of aconventional design, about one-third more resonance equivalentcapacitance, slightly less than twice the radiation resistance and abouttwice the motional inductance of the conventional design. These valuessuggest that different impedance inverter circuitry may be required inorder to employ the new design to greatest advantage.

                  TABLE I                                                         ______________________________________                                        Modeled characteristics of LiNbO.sub.3 resonators                             having an area of 4.5 × 10-8 m.sup.2, Cs = 3.969 pF, where the          new design incorporates an air gap of 0.1 micrometer                          between one electrode and the crystal.                                               Cm      Lm     Rm        Co    Cr                                             10-3 pF nH     Ω   pF    pF                                      ______________________________________                                        Conventional                                                                           8.88      1099   0.356   1.365 152.7                                 New Design                                                                             4.925     1978   0.6408  1.0107                                                                              204.8                                 ______________________________________                                    

EXAMPLE II

A series of calculations were made of C_(o) (static capacitance, givenin Pf), C_(m) (motional capacitance in 10⁻³ pF), R_(m) (motionalresistance), L_(m) (motional inductance in nanohenrys), fs (zerofrequency in gigahertz), fp (pole frequency in gigahertz), Qu (unloadedQ) and Cr (capacitance ratio) for LiNbO₃ resonators having a thickness dof circa 11.45 micrometers (operating at the N =5^(TH) harmonic) and anarea (of electrode 157) of 4.5·10⁻⁸ m² for different configurations.These values are summarized in Table II below.

                  TABLE II                                                        ______________________________________                                        Representative values for four different                                      resonator configurations illustrating effect of                               number of electrodes and choice of electrode                                  material on impedance characteristics. 1 = no                                 electrodes; 2 = 0.2 μm Al electrodes each side; 3 =                        0.1 μm Al electrodes each side; 4 = 0.1 μm Al on                        one side, 0.1 μm Au on the other; 5 = 0.1 μm Al                         electrodes on both sides; 1-4, d = 1.45                                       micrometers; 5, d = 11.3 micrometers.                                         1           2        3        4      5                                        ______________________________________                                        C.sub.o                                                                              1.356    1.356    1.356  1.356  1.3789                                 C.sub.m                                                                              8.88     8.75     8.87   7.93   8.99                                   R.sub.m                                                                              0.356    0.537    0.443  2.1785 0.438                                  L.sub.m                                                                              1099     1163     1123   1332   1080                                   fs     1.6104   1.57704  1.59355                                                                              1.54843                                                                              1.61448                                fp     1.6157   1.58213  1.59877                                                                              1.55295                                                                              1.61977                                Qu     31237    21453    25417  5951   25008                                  Cr     153      155      153    171    153                                    ______________________________________                                    

The results summarized in Table II show that the unloaded Q is grosslyreduced by use of a gold electrode (example 4) and that adding any metalreduces the unloaded Q (compare example 1 to the others). The goldelectrode also shows the greatest reduction in frequency.

CONCLUSION

Thus, a high Q resonator 15 has been described which overcomes specificproblems and accomplishes certain advantages relative to prior artmethods and mechanisms. The improvements over known technology aresignificant. The expense, complexities and high weight, size and powerrequirements of prior art approaches are avoided. Resonator 15 having athin gap between resonator layer 150 and second electrode 159 may beusefully combined to form filters and oscillators that aremanufacturable and are robust in operation, and may be combined withmonolithic integrated circuitry to provide signal processing modulesthat are extremely compact.

Moreover, in alternative embodiments (not illustrated), conductor 215(FIG. 4) may be formed on substrate 110 or may comprise a suitably-dopedand shaped region within substrate 110. These embodiments are not asefficient in heating resonator layer 150 but have the advantage ofrequiring less lithography to be performed on resonator layer 150.Alternatively, conductor 215 may be configured as a single strip alongone or more sides of electrode 157. This arrangement simplifies layoutetc. but also introduces undesirable temperature gradients withinresonator layer 150.

It will also be appreciated that separating bottom electrode 159 fromresonator layer 150 allows conductor 215 (e.g., a loop surrounding theregion occupied by electrode 157) that is deposited on resonator layer150, to be included on the lower surface thereof, obviating need forcrossovers or other structures that obtains when electrode 157 isdisposed on the same surface of resonator layer 150 as is conductor 215.

It will be appreciated that either of the welding techniques describedmay be employed with either of the gas-phase etching processes describedfor thinning resonator layer 150 or that other bonding or thinningprocesses may be usefully applied for manufacturing acoustic resonator15.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingcurrent knowledge, readily modify and/or adapt for various applicationssuch specific embodiments without departing from the generic concept,and therefore such adaptations and modifications should and are intendedto be comprehended within the meaning and range of equivalents of thedisclosed embodiments.

It is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.Accordingly, the invention is intended to embrace all such alternatives,modifications, equivalents and variations as fall within the spirit andbroad scope of the appended claims.

I claim:
 1. A high quality factor resonator comprising:a substratehaving a surface; a resonator layer having first and second surfaces andincluding a first electrode, said resonator layer disposed on saidsurface, said first electrode disposed on said first surface of saidresonator layer, said first surface being more remote from saidsubstrate than said second surface; a cavity disposed between saidsubstrate and said resonator layer; and a second electrode disposed onsaid substrate and on a surface of said cavity remote from saidresonator layer, wherein said second electrode is separated from saidresonator layer by a thin gap.
 2. A high quality factor resonator asclaimed in claim 1, wherein said resonator layer comprises asingle-crystal material.
 3. A high quality factor resonator as claimedin claim 1, wherein said resonator layer comprises lithium niobate,lithium tantalate or lithium tetraborate.
 4. A high quality factorresonator as claimed in claim 1, wherein said resonator layer comprisessingle-crystal lithium niobate, lithium tantalate or lithiumtetraborate.
 5. A high quality factor resonator as claimed in claim 1,wherein said first electrode includes aluminum.
 6. A high quality factorresonator as claimed in claim 1, wherein first electrode issubstantially circular.
 7. A high quality factor resonator as claimed inclaim 1, wherein said substrate includes a semiconductor substratecomprising an integrated circuit.
 8. A high quality factor resonator asclaimed in claim 1, wherein said second electrode comprises gold.
 9. Ahigh quality factor resonator as claimed in claim 1, wherein saidresonator layer is welded to said substrate.
 10. A high quality factorresonator comprising:a substrate comprising a semiconductor material andincluding an integrated circuit; a resonator layer comprising apiezoelectric material and including a first electrode, said resonatorlayer disposed on a first surface of said substrate; a cavity disposedbetween said substrate and said resonator layer; and a second electrodedisposed on said substrate and on a surface of said cavity remote fromsaid resonator layer.
 11. A high quality factor resonator as claimed inclaim 10, wherein:said resonator layer comprises single-crystal lithiumniobate, lithium tantalate or lithium tetraborate; and said resonatorlayer is welded to said substrate.